1. Field of the Invention
The present invention relates to motor drive circuits and motor drive methods, and more particularly, to a motor drive circuit having a short brake function and a motor drive method capable of performing a short brake operation.
2. Description of the Related Art
Motor drive circuits are circuits that supply a drive current to a motor for rotating the motor. In such a motor drive circuit, when the supply of the drive current to the motor is stopped in a state where the drive current is supplied to the motor and the motor is being rotated, the motor tries to continue rotating itself by inertia. When the motor is rotated by inertia, a counter electromotive force is generated in the motor in accordance with the rotation. The counter electromotive force generated in the motor is applied to the motor drive circuit. Accordingly, the longer the time interval of rotation of the motor is and the faster the rotation is, the longer the time interval of application of the counter electromotive force becomes and the greater the counter electromotive force becomes.
Motor drive circuits are circuits for supplying a drive current to a motor. Thus, when a counter electromotive force is applied to a motor drive circuit by a motor, there is a possibility of a malfunction. In order to avoid such a malfunction, some motor drive circuits are provided with a brake function that forces the motor to stop rotating.
The brake function of a motor drive circuit includes a function referred to as short brake. Short brake is a function that, when the supply of a drive current from a motor drive circuit to a motor is stopped, brakes the motor by forming a loop connecting both poles of the motor and regenerating a current generated by a counter electromotive force.
FIG. 1 is a block diagram of a motor drive circuit.
A motor drive circuit 100 shown in FIG. 1 has an H bridge circuit configuration and includes a drive circuit 112 and output transistors Q111 through Q114.
The emitter of the output transistor Q111 and the collector of the output transistor Q112 are connected in series between a power supply voltage Vcc and the ground. The emitter of the output transistor Q113 and the collector of the output transistor Q114 are connected in series between the power supply voltage Vcc and the ground. Switching control is performed on the output transistors Q111 through Q114 by the drive circuit 112.
When a motor 111 is to be rotated in the right direction, the output transistors Q111 and Q114 are turned ON and the output transistors Q113 and Q112 are turned OFF by the drive circuit 112. On the other hand, when the motor 111 is to be rotated in the reversed direction, the output transistors Q113 and Q112 are turned ON and the output transistors Q111 and Q114 are turned OFF by the drive circuit 112 (refer to Japanese Laid-Open Patent Application No. 8-154396, for example).
In conventional motor drive circuits, the output transistor Q111 is turned OFF and the output transistor Q112 is turned ON at the time of short brake. On this occasion, the potential of a connection point between the output transistors Q111 and Q112 is increased by a counter electromotive force generated in the motor 111. When the potential of the connection point between the output transistors Q111 and Q112 is increased, the timing at which the output transistor Q111 is turned OFF is delayed compared to the timing at which the output transistor Q112 is turned ON. Consequently, a time interval occurs during which the output transistors Q111 and Q112 are simultaneously ON.
When the output transistors Q111 and Q112 are simultaneously ON, a current flowing from a power supply to the ground via the output transistors Q111 and Q112, i.e., a shoot-through current, flows. When the time interval during which the shoot-through current flows through the output transistors Q111 and Q112 is long, there is a problem in that a malfunction occurs in the operation of the drive circuit 112, for example.